1. Field of the Invention
The present invention relates to a battery cover member that has a cover member through which electrode terminals penetrate and that seals an opening of a battery container.
2. Description of the Related Art
A lithium ion secondary battery that has high capacity, produces a high power, and also exhibits high energy density has recently received attention as an electronic power source for a portable electronic device and a power source for a pure electric vehicle.
The lithium ion secondary battery is built by sealing a roll electrode member serving as a power generation member in a battery container along with a non-aqueous electrolyte. Incidentally, if moisture enters the battery container, the moisture will react with the non-aqueous electrolyte, thereby generating a fluorinated acid. The thus-generated fluorinated acid eats into electrodes, which in turn sometimes results in deterioration of battery performance, such as battery capacity and battery life.
For these reasons, a battery container of the lithium ion secondary battery is formed from metal so as to prevent entry of moisture into the battery container. The metallic battery container must keep a sealing characteristic between an interior and an exterior of the battery container. The battery container is configured such that clearance between a cover member and electrode terminals penetrating through the cover member, to thus project to the outside from the interior of the battery is sealed by use of an O-ring.
However, a sealing structure using an O-ring entails a large number of components and, hence, poses a problem in terms of productivity and cost. In order to solve such a problem, Patent Document 1 provides a proposal of a battery cover member that bonds an upper battery cover to electrode terminals by means of a resin sealing insulation member, thereby combining the upper battery cover and the electrode terminals in one unit and that ensures a sealing characteristic between the interior and the exterior of the battery. As shown in FIG. 8, the battery cover member 110 is made up of a cover member 130 and a pair of electrode terminals 140, and the pair of electrode terminals 140 inserted into through holes made in the cover member 130 are assembled into a mold. A space between the cover member 130 and the electrode terminals 140 are filled with a molten resin insulating sealing member 150, thereby integrating the cover member and the electrode terminals into one unit by means of insert molding. Since the cover member 130 and the electrode terminals 140 are hardly bonded into one unit by way of the resin insulating sealing member 150, the battery cover member 110 exhibits a high sealing characteristic.
However, metal and a resin differ from each other in terms of a coefficient of thermal expansion. Therefore, stress develops in an adhesive interface between the metal and the resin in a high-temperature or low-temperature environment, which in turn impairs adhesion of the electrode terminal 140 to the resin insulating sealing member 150, induces cracks in the resin insulating sealing member 150, and fails to assure the sealing characteristic of the battery cover member 110. Since the electrode terminals 140 of the battery cover 110 assumes a rectangular cross sectional profile as shown in FIG. 8, a corner is particularly likely to become a starting point of cracks due to stress concentration. Further, in a case where electrode terminals are subjected to surface treatment in order to enhance adhesion, when the electrode terminal has a rectangular cross section, generating a coating film by means of surface treatment changes from a linear portion to an angular portion, which makes it difficult to subject an entire adhesion surface between the electrode terminal and an insulation sealing member to uniform surface treatment over the entire circumference. Furthermore, since an adhesion area includes both linear and angular portions, there also arises another problem of operation of an inspection apparatus becoming difficult.
Patent Document 1: JP-A-2008-27823
The present invention has been conceived in light of the above-mentioned circumstance, and a challenge to be met by the present invention is to provide a battery cover member into which electrode terminals and a cover member are integrally bonded into a single unit by means of a corresponding insulating resin member and that exhibits superior adhesion between electrode members and the insulating resin member.